Recent studies indicate that only 50% of the heme degraded by the liver is processed to equimolar amounts of bilirubin and carbon monoxide. The metabolic fate of the remaining heme is not known, although it has been suggested that cytochrome P-450 heme, which accounts for over 40% of hepatic heme turnover, may be degraded by pathways not involving bilirubin and CO production. NADPH cytochrome P-450 reductase dependent oxidative turnover of cytochrome P-450 in vitro has been shown to be accompanied by the formation of an uncharacterized mixture of dipyrrolic heme fragments known as propentydopents and by the attachment of heme fragments to cytochrome P-450 apoprotein. This autodegradation reaction could represent the missing heme metabolic pathway and also serve as a means to signal cytochrome P-450 turnover. In order to elucidate the mechanism of this reaction in vitro and to assess its importance in vivo, we have synthesized the major tetra-, di-, and monopyrroles expected from oxidative heme cleavage and developed methods to assay for them.